If you liked Doom 3's singleplayer and you really want a few more hours of the mindless fun involved...then here it is.
Resurrection of Evil is an unspectacular add-on to the game, and won't give you much more then another 5-6 hours of typical Doom 3 gameplay walking from room to room takin' down monsters from hell. It does have some nice additions to the gameplay with the "gravity gun" and the whole slow motion thing they added, but it's still really missing the online coop the last Xbox version had.
So if you liked Doom 3's singleplayer and you really want a few more hours of the mindless fun involved, then its an OK pickup...otherwise don't expect any great improvements.